1. Field of the Invention
The present invention relates to an image scanning device, and more particularly, to an image scanning device having two parallel wheel sets for maintaining scanning direction.
2. Description of the Prior Art
Please refer to FIG. 1 and FIG. 2. FIG. 1 is a top sectional view of a prior art flatbed scanning device 10. FIG. 2 is a sectional view taken along line 2--2 of the scanning device 10. The scanning device 10 comprises a housing 11, a scanning module 20, and a driving device which comprises steel strips 16. The housing 11 comprises a transparent platform 12 on its top for placing a document to be scanned, and two parallel tracks 14 installed along left and right directions below the transparent platform 12. The scanning module 20 is slidably mounted on the parallel tracks 14. It comprises a plastic chassis 28, an elongated image sensor 18 mounted inside the chassis 28 for scanning the document, and two wheel sets 21 installed under two ends of the chassis 28 for engaging the parallel tracks 14 and supporting the chassis 28 upward. Each wheel set 21 comprises a roller wheel 22 installed on its lower end for rotatably engaging one track 14, and a spring (not shown) inside for elastically pushing the chassis 28 upward against the transparent platform 12. The steel strips 16 of the driving device are mounted on two ends of the chassis 28 for pulling the scanning module 20 along the two parallel tracks 14 of the transparent platform 12. The wiring structure of the steel strips 16 is quite complex and is only partially shown on FIG. 1.
Since each wheel set 21 of the scanning module 20 has only one roller wheel 22, it can neither properly maintain the sliding direction of the scanning module 20 nor can it keep the balance of the scanning module 20 on the tracks 14 when scanning a document. The sliding direction and balance of the scanning module 20 are completely maintained by the wiring structure of the steel strips 16. However, the wiring structure of the steel strips 16 is very complex and it is very time consuming to install or adjust the wiring structure of the steel strips 16.